1. Field of the Invention
This invention relates to trimmable resistor networks especially for use as a part of an integrated circuit (IC). More particularly, this invention relates to such networks of the type comprising a number of individual links which can be selectively cut to change the resistance presented by the network.
2. Description of the Prior Art
Integrated circuits often require resistors the resistance values of which can be trimmed over a wide range. For example, such resistors may be needed to adjust the net resistance of a circuit resistor having an initial tolerance of .+-.20%, to an absolute tolerance less than a fraction of a percent. Even wider trim range is necessary to ratio-match resistors having different compositions and tolerances as great as .+-.30%. In ratio matching, the trim range must accommodate the sum of both sets of tolerances. Other cases arise in practice which also require substantial trim range.
A large trim range can be provided by an individual resistor which is controllably cut through a major portion of its area. Such a technique is however unsatisfactory in many respects. Large trim range resistors are often very large in area, and require long and time consuming trims which makes the parts (ICs) expensive to manufacture. These long trims produce a large "wound" and surrounding the completely trimmed kerf is a partially trimmed transition region which is less stable than the untrimmed portion of the resistor. The wounded area typically exhibits a slightly different temperature coefficient than the untrimmed portion of the resistor. If a resistor is trimmed over a long portion of its length, the relatively larger wound may seriously affect stability and temperature tracking.
Some of these problems can be overcome by use of link trimming. In this approach, a resistor is constructed so that it has a number of alternative conduction paths through links which may be selectively cut. Generally, when one of these links is cut, the path is opened and no current flows in the cut region. This makes the trimmed resistor essentially free of the stability and TC effects produced by wounds in a large-area trim resistor.
One type of link network comprises a pair of parallel conductors having a series of link resistors connected between the conductors, somewhat like ties of a railroad track. By cutting many links, a large change in resistance can be obtained. One last link can be partially cut to make a fine adjustment in the resistor values. This one cut can have a small area so as to have only a minor effect on stability or TC.
This railroad track design has a number of limitations. Because all of the links are in parallel, a large width must be trimmed, requiring long trim times. Such a resistor network also is relatively large and inefficient in terms of trim linearity. The effect of cutting an individual link depends upon the number of links already cut. If all links are of equal width, the first few links cut have a much smaller effect on overall resistance than the effect of the last few after many have been cut. This is opposite to what is desired, i.e., a large coarse trim followed by finer high-resolution trimming.
To overcome these deficiencies, modifications have been proposed to the railroad track design, such as using links of different sizes which are selectively cut to achieve predetermined changes in resistance. Many of these modification schemes suffer from the weakness that although a large number of resistance values can be obtained, many of these values are redundant, or may not be useful because they are so far away from other values that there is no practical way to interpolate between them. Thus, the actual useful range and resolution of such a design is much less than the range of possible resistance values or the resolution of the smallest change.